1. Field of the Invention
This invention relates to a process for separating a mixture of wax and mineral oil. More particularly, it relates to an improved solvent dewaxing process.
2. Description of the Prior Art
It is well known in the art to remove waxy constituents from wax-containing hydrocarbons, particularly from wax-containing petroleum oils by various methods. These processes generally chill the wax-containing oil in the presence of a solvent to a temperature at which the waxy constituents are crystallized (precipitated) out of solution. The chilled mixture containing the crystallized wax is then further treated to separate the crystallized wax particles from the dewaxed oil by various means usually by filtration, although sedimentation or centrifugation may be used.
It is known to dewax oil by DILCHILL (service mark of Exxon Research and Engineering Company for a dewaxing process) such as the process described in U.S. Pat. 3,773,650 issued Nov. 20, 1973; U.S. Pat. No. 3,644,195 issued Feb. 22, 1972 and U.S. Pat. No. 3,642,609 issued Feb. 15, 1972, the teachings of which are hereby incorporated by reference. The DILCHILL process comprises introducing a wax-oil mixture containing a substantial portion of wax dissolved therein into a cooling zone divided into a plurality of stages and passing the wax-oil mixture from stage to stage of the cooling zone while introducing cold dewaxing solvent incrementally along the length of the cooling zone thereby cooling the wax-oil mixture and precipitating a substantial portion of the wax therefrom. High levels of agitation are provided in at least a portion of the solvent-wax oil mixture-containing stages thereby providing substantially instantaneous mixing of the oil and solvent. Since utilization of the DILCHILL technique to cool the mixture completely to a subsequent wax separation temperature (e.g., filtration temperature) requires a high solvent dilution ratio or very low solvent temperatures which are obtainable, for example, by using a cascade refrigeration system, it has been found preferable to utilize the DILCHILL process to reduce the temperature of the waxy oil only partially to a temperature above the wax separation temperature followed by cooling in an additional chilling stage, such as, for example, the combination DILCHILL with scraped surface chilling process described in U.S. Pat. No. 3,775,288 issued Nov. 27, 1973, the teachings of which are hereby incorporated by reference.
It has now been found that the waxy oil mixture can be chilled to the wax separation temperature or to a temperature less than about 25.degree.F. above the wax separation temperature without the above stated disadvantages.